Research is in progress to determine the relationships between the structure and delayed neurotoxicity in a group of halogenated phosphonate, phosphate, and phosphoramidate esters related to leptophos O-(2,5-dichloro-4-bromophenyl) O-methyl phenylphononothionate. These compounds are being evaluated in hens by feeding single and multiple oral dosages. The enzymology and histopathology of the central nervous system lesion produced by the neurotoxic compounds is under concurrent evaluation. Related studies are exploring the degradative fate of selected radiolabeled neurotoxic phosphonothionate esters in mice, chicks, and in laboratory model ecosystems.